This invention relates to control apparatus for electron beam lithography systems and, more particularly, to control apparatus for high speed pattern generation in a system utilizing variable length line scanning.
Electron beam exposure systems are used commercially for selectively irradiating a resistcoated workpiece to define the features of a semiconductor device. The workpiece can be a mask plate or can be a resist-coated semiconductor wafer in which features are defined directly. In either case, an electron beam is controlled in a highly accurate, high speed manner to expose complex microminiature patterns in the electron resist material. The minimum feature dimensions can be less than one micrometer. As semiconductor devices with greater complexity are developed, the desire is to reduce pattern dimensions even further.
In order to expose these microminiature patterns, the electron beam is focused and shaped to the required dimension and applied to the electron-sensitive resist. In addition, the beam is deflected to prescribed pattern locations on the workpiece. A variety of shaping and deflection techniques have been utilized. A small Gaussian spot beam can be raster-scanned over the entire surface of the workpiece and turned on or off to produce a desired pattern. A system using this approach is disclosed in U.S. Pat. No. 3,900,737, issued Aug. 19, 1975, to Collier et al. Alternatively, the spot beam can be directed to desired pattern areas and scanned only over those pattern areas in a vector scanning approach. Either approach is relatively slow, since the area covered by the spot beam at any instant is extremely small. In another system, the electron beam is shaped into rectangles of variable shape and size. Successive areas of the pattern are flash exposed with the rectangular beam. One drawback of variable shaped rectangle systems is the difficulty in exposing angled or odd shaped pattern features.
A technique known as variable line scanning has been developed to provide high speed operation and the capability to easily expose arbitrarily shaped pattern features. A beam having a rectangular cross-section is scanned in a direction perpendicular to its long dimension. As the beam is scanned, the length of the rectangle is varied to define a desired pattern feature. In producing a variable shaped beam, an image of a first aperture is focused on a second square aperture. The first aperture can be square or L-shaped. Shaping deflectors dynamically position the image of the first aperture relative to the second aperture so as to provide a beam cross-section of the desired length and width. Additional deflectors position the beam on the workpiece and scan the rectangular beam in the desired direction.
As integrated circuits become more complex, the number of devices per chip increases; and the size of the individual devices is decreased. In writing device patterns with an electron beam exposure system, each device is broken down into a number of elementary shapes, such as rectangles and trapezoids, which can be easily described and which are more easily written by the electron beam system. However, this breaking up of pattern features increases the number of individual features to be written by the system. Furthermore, due to the proximity effect resulting from backscattering of electrons in the resist material, it is common to partition pattern features into subfeatures and to expose the subfeatures with variable dosage depending on their size and proximity to other pattern features. This results in a still further increase in the number of individual pattern features to be exposed by the system. The result is that an integrated circuit device may require on the order of a million or more individual pattern features. Such a pattern represents only one of several pattern layers in the complete integrated circuit device. Therefore, the speed at which individual pattern features are exposed by the system is a matter of utmost importance in commercial semiconductor processing. In addition, the accuracy of the patterns must be maintained in order to insure proper operation of the complex devices.
The data representing each of the pattern features is typically stored in digital form on a mass storage device, such as a magnetic disc or tape, and is read as required by the electron beam exposure system. The digital data must be converted to analog signals for energizing the shaping and deflection elements of the electron beam column. In order to achieve high speed lithographic writing in a system utilizing variable line scanning, the conversion of digital data to analog signals must be performed in an efficient and high speed manner.
It is a general object of the present invention to provide novel apparatus for performing charged particle beam lithography.
It is another object of the present invention to provide novel apparatus for high speed variable line scan charged particle beam lithography.
It is yet another object of the present invention to provide apparatus for controlling the beam shape and beam position in a charged particle beam lithography system.
It is still another object of the present invention to provide novel apparatus for converting data representing location, size and shape of polygon features into signals for controlling the shape and position of a charged particle beam.